The present invention relates to a communication method for providing a distribution service by multicast, a communication system, and an apparatus for relaying data packet distributed by multicast.
It has been increasing the building of a wireless network which connects terminals provided with a wireless communication function and mobile telephones which are capable of data communication have been spreading widely. Mobile telephones have become able to receive music and dynamic image, and the chances to distribute information to information apparatuses and terminals which are capable of mobile communication will increase in the future. The multicast communication is a communication method which broadcasts data packet to a large number of terminals, and generally can distribute information efficiently compared to the unicast communication performed between the terminals. The multicast communication is suited to distribute in particular information which occurs in real time, for example, live broadcast, sensing information in various environments and monitoring information of status.
In the multicast communication, a sending terminal sends a data packet addressed to a multicast group. On the other hand, a receiving terminal can receive the data packet by becoming a group member of a desired multicast group, and can stop receiving data packet by withdrawing from the group member.
The receiving terminal uses a group management protocol and sends a control packet which makes a group attend request to become a group member or a group withdrawal request to withdraw from a group member to a router connected to the same network. The receiving terminal is able to request to attend and withdraw from the multicast group member at any time and freely. As a group management protocol IGMP (Internet Group Management Protocol) is defined by standard recommendation document RFC (Request For Comments) 2236 created by IETF (Internet Engineering Task Force), and by RFC2710 MLD (Multicast Listener Discovery) is defined.
A router periodically sends an attend group inquiry packet by the group management protocol to check whether a group member exists or not, and the receiving terminal being a group member responds to the inquiry by a group attend request. The router decides whether a group member exists or not by the presence of the response to the inquiry packet, and relays data packet only to networks in which group members exist. The router confirms the existence of group member, and when it decides that relay of data packet is necessary, posts a distribution request to other routers using a multicast path control protocol. Alternatively, when group member does not exist and it decides that relay of data packet is not necessary, it posts a distribution stop request. Each router retains relay information for multicast data packet, and updates the relay information according to the contents of the posted multicast path control protocol, and further, sends a distribution request or distribution stop request toward a sending terminal according to the update contents. The distribution request or distribution stop request is processed by a multicast router existing on the path from the sending terminal to each receiving terminal, and the best suited multicast distribution path is established from the sending terminal to each receiving terminal. As a typical multicast path control protocol, PIM-SM (Protocol Independent Multicast Sparse Mode) is defined by RFC2362.
To provide a multicast distribution service it is necessary that the above-mentioned multicast path control protocol would operate on all of the routers which configure the network. Also, on the side of the terminal which receives data packet distributed by multicast the group management protocol must operate. This means that it is needed to add functions to all the existing apparatuses and it would be highly costly.
On the other hand, as a communication technique for a mobile terminal which communicates while moving such as a mobile telephone mobile IP is defined by RFC2002. The mobile terminal uses an address called a home address on the original connection network (hereinafter it is called a home network). A home agent (hereinafter it is called HA) which is located in each network manages the home address of the mobile terminal. When the mobile terminal moves to other network, it is given a care-of address by a foreign agent (hereinafter it is called FA) which is located in the moving destination network (hereinafter it is called a foreign network), which it can use only while it is connected to the foreign network. The mobile terminal first sends a registration request message including the given care-of address to FA to register the care-of address in HA, and FA transfers it to HA. On receiving the registration request message, HA retains the care-of address and sends a registration response message which indicates whether the registration request has been accepted or rejected to FA or the mobile terminal. Hereafter, on receiving a data packet addressed to the home address of the mobile terminal, HA encapsulates and transfers the data packet to the care-of address. On receiving the encapsulated data packet, FA decapsulates the data packet and transfers it to the mobile terminal. Although in the mobile IP a data packet addressed to the mobile terminal passes via HA, a packet which the mobile terminal sends reaches directly to the corresponding terminal. HA and FA send an agent advertisement message to decide whether the mobile terminal is connected to the home network or to the foreign network. When FA does not exist in the foreign network, the mobile terminal can post the registration request message directly to HA and HA can receive directly the encapsulated data packet.
In the mobile IP two methods are defined for the mobile terminal to receive multicast data in the foreign network. One is a method to send a group attend request to HA in which when HA receives multicast data addressed to the group which the mobile terminal requested, it encapsulates and sends the packet to the care-of address. Another is a method to issue a group attend request to a multicast router located in the foreign network. When performing multicast by this mobile IP, the mobile terminal and HA or the router located in the foreign network need to be provided with a multicast function.